Photo electric cell circuits



March 19, 1935. G. M. WRIGHT ET AL 1,995,138

PHOTO ELECTRIC CELL CIRCUITS Filed Aug. 10, 1929 Drum INVENTORs GEORGE MAumcE wmem' m BY 5mm BERTRAM SNHTH Patented Mar. 19, 1935 P r r 1,995,138 N PHOTO ELECTRIC cam. oinourrs George Maurice Wright, Burford, and Sidney Bertram Smith, Gordonville, England, assignors to Radio Corporation of America, a corporation of Delaware Application August 10, 1929, Serial No. 384,884

in Great Britain August 31, 1928 y scams. (or. 250- 4 .'5)

This invention relates to photoelectric .cells, and circuits therefor, and more particularly to photoelectric cell circuit arrangements for use in picture telegraphy and for like purposes.

It is well known that to obtain satisfactory and efiicient amplification of the output of a photoelectric cell employed, for example, in a picture telegraph transmitter, it is strongly desirable,-and in fact practically necessary, that the output potential'be of intermittent or alternating character.

In known apparatus, this intermittence or alternation has been achieved by superimposing an alternating potential in the cell circuit or by interrupting the light, whereby the cell is activated.

According to this invention, means for rendering the output of a photoelectric cell of intermittent character comprise a source of polarizing potential, a condenser in series therewith and with said cell, and means for intermittently shortcircuiting said condenser.

In one form of construction, a photoelectric cell is connected in series with a battery and a condenser, and the grid filament space of a thermionic valve amplifier is shunted across the condenser, which is also shunted by a periodic short-circuiting device.

The ratio of the time during which the condenser is short-circuited to the time during which it is allowed to be charged through the cell from the source of polarizing potential should be adjusted so as to give maximum output from the thermionic valve amplifier, and if desired the duration of the short-circuiting period of the short-circuiting device may be made adjustable so as to give ready control of this ratio.

Where the invention is applied to picture telegraph apparatus, the frequency of short-circuiting of the condenser should be such as to give a suitable grain to the picture as received; a frequency giving a picture grain of dots per inch has been found satisfactory.

Preferably also the frequency of short-circuiting is a definite multiple of the speed of the scanning light beam by which the picture to be transmitted is explored in most picture telegraph transmitters as at present employed, since with such relationship between short-circuiting frequency and scanning speed the modulation frequency and the grain of the received picture will be independent of the rate of scanning. This relationship may conveniently be obtained by employing as the short-circuiting device a commutator or the like, driven in common with the drive for the scanning apparatus in the picture transmitter. .2 1

The invention is illustrated in the accompanying drawing,- which shows one form of construction'in accordance therewith.

Referring to the figure,- 1 is a battery, 2 a photoelectric cell and 3 a condenser of high insulating properties. 5 is a, triode whose gridfilament space is'shunted across the condenser 3 and whose output circuit is-jcoupled or'connected in any suitable manner to an amplifier 6.

Also shunted across the condenser 3 is'a shortcircuiting commutator 4 of very high insulation resistance initsopen circuit positions, i. e., when the condenser is charging, and as nearly as possible zero resistance in its closed circuit positions, i. e., when the condenser is discharging. The circuit is so adjusted as regards the potential of the battery 1 and the cell 2 that no grid current flows in the valve 5 so that there is no shunt leakage across the condenser 3. r

The device 4 may be constituted by a rotary commutator driven in any convenient way. For example, in picture telegraph apparatus the device 4 may conveniently be geared to the synchronous motor normally provided for driving the scanning or traversing mechanism or it may be driven by a separate synchronous motor energized from the same source as that which energizes the said scanning and traversing motor. It will be seen that with such an arrangement the rate of charge and discharge of the condenser 3 can be made to have any desired ratio to that of the picture scanning or exploring device, while the charge received by the condenser will be proportional to the time interval between successive. chargings and dischargings. Thus, if the speed of the scanning mechanism be reduced by one half, the speed of the device 4 will be similarly reduced, the charge receivedby the condenser 3 will be correspondingly increased and the photo-' electric cell response will be doubled.

When the available light intensities for actuating the cell 2 are weak, practical difficulties may be experienced with certain forms of mechanical commutators or the like if they be employed to constitute the device 4, owing to the high degree of amplification, which will be required from the amplifier 6. Contact potentials and microphonic noises may create an objectionably high noise level at the input to the triode 5 with consequent mutilation of the required signals. In such cases any known form of thermionic short-circuiting device or other substantially noise-free short-circuiting device may We have merely attempted to suggest a certain form which the invention might assume, although it is to be understood that this has been illustrated merely by way of example, and is to be regarded as generic and not limiting, and it should be readily understood that certain modifications and changes will at once suggest themselves to those skilled in the art to which this invention relates, and we therefore desire to be limited in what modifications we canrnake only in so far as the scope of' the hereinafter appended claims necessitates.

Having now fully described and illustrated our invention, we claim: 1

1. An apparatus for introducing a chopper frequency into the output of a photoelectric tube which comprises a closed network including a source of potential, at photoelectrictube. and a condenser each connected in series with one another, a thermionic valve having its input circuit connected across the network and the electrodes of said condenser so as to amplify the photoelectric tube output currents, and a commutator means connected across said condenser and the input circuit of said thermionic amplifier for pcriodically short-circuiting said condenser. and supplying the photoelectric tube currents to the amplifier as alternating currents. I

2. In an apparatus for rendering the output from photoelectric tubes of intermittent character, a photoelectric tube, a condenser and a source of potential for said tube; said tube, condenser and source being connected in series one with the other and forming a closed network, a thermionic tube connected across said condenser for amplifying the produced photoelectric currents, commutator means connected across the input circuit of said thermionic amplifier and across said condenser element for intermittently permitting said condenser to charge and to discharge so as to supply photoelectric cell currents to the amplifier as alternating currents, and a means for driving said commutator at a speed commensurate with the rate at which successive elemental areas of the subject of which the image is desired influence the tube for maintaining a predetermined rate between the analyzing speed and the rate of interruption of the photoelectric tube currents.

3. In a communication system, a light translating. device, a capacity element and. a source of electrical energy connected in series with each other and with the light translating device whereby the capacity element is charged at a rate proportional to the intensity of light reaching the light sensitive device, a thermionic amplifier device having its input circuit connected across said capacity element, and a cyclically operating short-circuiting device connected across said tube input circuit for intermittently permitting the condenser to charge and discharge so as to supply photoelectric currents to the amplifier as alternating currents.

GEORGE MAURICE WRIGHT. SIDNEY BERTRAM SMITH. 

